programs such that pressing a button on the Arduino causes the attached PC to open a file
You might as well be asking how hard you have to pedal to get your bicycle into orbit.
Arduino is good for processing simple and small text files.
i think there is a misunderstanding here.
I will tell you what exactly needed.
More of a "no-"understanding, since you gave insufficient info.You should have led with that....
Button 1 will open a word file.Button 2 will open an excel file.
There is a spare parts store need to give RFID tag to each customer.And when he read this tag, simply an excel file for this tag "or customer" opened in the computer so he can edit it.